The present invention is directed to improved woodworking adhesives prepared from multistage emulsion polymers, the adhesives being characterized by a superior degree of water resistance.
Aqueous emulsions of vinyl acetate homo- or copolymers are widely used in formulating wood adhesives wherein bond strength, fast setting, colorless glue lines and ease of application are desired. In order to improve water resistance and other properties of these adhesives they are generally formulated with other resins such as resorcinol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, phenol-formaldehyde resins and trimethylol phenol oligomer or with fusion aids, such as tetrahydrofurturyl alcohol or ethylene glycol diacetate. These preformulated adhesives may then be catalyzed with various acidic metal salts as is known in the art. These adhesive systems are described, for example, in U.S. Pat. Nos. 3,301,809; 3,563,851, 3,433,701; 3,041,301; 4,474,915; and 4,687,809.
While such formulated vinyl acetate based adhesives provide adequate water resistance for plywood and finger jointed lumber constructions tested at room temperature, under vacuum-pressure soaks and in boiling water, none of the current vinyl acetate adhesives meet ASTM standards for exterior lumber, especially for ASTM D-3110 for edge glued lumber. This is illustrated by the requirements for these tests on pine lumber, as well as the values obtained using conventional formulated crosslinked polyvinyl acetate adhesives (PVA) presented in Table I.
TABLE I ______________________________________ PRESSURE ROOM TEMP. SOAKS BOILS ______________________________________ FINGER-JOINTED PINE Typ- 4498 psi, 96% WF 2400 psi, 68% WF 2200 psi, 60% WF ical PVA Re- 2000 psi, 60% WF 1600 psi, 50% WF 1600 psi, 50% WF quire- ments EDGE GLUED PINE Typ- 1411 psi, 97% WF 275 psi, 20% WF 300 psi, 15% WF ical PVA Re- 678 psi, 60% WF 565 psi, 50% WF 565 psi, 50% WF quire- ments ______________________________________